1. Field of the Invention
The invention pertains to a three-electrode electrochemical sensor for the detection of atmospheric oxygen.
2. Description of Related Art
Most oxygen sensors in the safety market are two electrode Galvanic cells with a working electrode made of a noble metal and a counter electrode made of lead (Pb). Examples of this type of sensor are disclosed in U.S. Pat. Nos. 4,132,616 and 4,324,632. These sensors have proven to be accurate and reliable for the measurement of the oxygen (O2) concentration in air; the sensors do not require long start-up times after they are installed into gas detection instruments, or after the instruments are powered back on, since the two electrodes can be xe2x80x9cshortedxe2x80x9d, or connected with a small load resistor when not in use. However, the counter electrodes used in these sensors are consumed over time in the presence of oxygen (2 Pb+O2xe2x86x922 PbO), and the sensors therefore have a very limited lifetime, 1 year being typical. Furthermore, lead is a chemical hazard and an environmental toxin, and disposal of used oxygen sensors has been an increasing environmental concern.
Non-consumable oxygen sensors can be made that incorporate a noble metal/air electrode as the reference electrode. Examples of this type of sensor are described in German patent DE 4231256 C2 and U.S. Pat. No. 6,024,853. This type of sensor has three electrodes, a working electrode, a reference electrode, and a counter electrode, with an aqueous electrolyte. The potential of the working electrode is controlled by a potentiostat with respect to the potential of the reference electrode, which is typically about +1.15 V versus the standard hydrogen electrode (SHE) in an acidic medium. The potential of the working electrode is maintained sufficiently negative that reduction of oxygen proceeds rapidly on the electrode surface. The overall electrode reaction is controlled by diffusion of oxygen to electrode surface and the sensor shows a steady state current that is in direct proportion to the concentration of oxygen in air. The electrochemical reaction at the working electrode can be expressed as:
O2 (WE)+4H++4exe2x88x92xe2x86x922H2Oxe2x80x83xe2x80x83(1)
Concurrently at the counter electrode:
2H2Oxe2x86x92O2 (CE)+4H++4exe2x88x92xe2x80x83xe2x80x83(2)
The net reaction in the cell is:
O2 (WE)xe2x86x92O2(CE)xe2x80x83xe2x80x83(3)
where O2 (WE) and O2 (CE) represent oxygen at the working electrode and at the counter electrode, respectively. Equation 3 shows that the three electrode sensor operates as an xe2x80x9coxygen pumpxe2x80x9d, i.e. when oxygen is reduced in the sensor at the working electrode, an equal quantity of oxygen is generated in another part of sensor where the counter electrode is located. As neither the electrodes nor the electrolyte is consumed, the sensor can be operated free of maintenance for many years.
A major problem with this type of sensor has been the balancing of air pressures in and out of the sensor cell. The counter electrode is one of the key components inside the sensor, and because of constant production of oxygen gas at this electrode, a high air pressure will develop quickly inside the sensor and particularly in the electrolyte reservoir where substantial free space is normally available to allow for electrolyte expansion. If the pressure inside the sensor increases sufficiently, then liquid electrolyte will start to leak out of the cell.
U.S. Pat. No. 6,024,853 describes a sensor in which the counter electrode is located in the center of the sensor cell, and a porous PTFE disk is used in the bottom to close the cell. This sensor is very prone to leakage because the PTFE disk is covered by an electrolyte absorbent material and by free electrolyte, which can provide a gas tight seal and thereby prevent the pressure build up in the sensor from safely venting through the PTFE disk.
In another three electrode oxygen sensor that is manufactured by Draegerwek, Luebeck, Germany, the sensor has a hole in the portion of housing where the electrolyte reservoir is located, in the middle of the cell. The hole is sealed with a tube that extends inside the sensor, the tube being formed of a hydrophobic gas permeable material which allows air exchange between the inside and outside of the sensor. There are, however, several drawbacks with this design. First, the gas permeable material must have large pores to facilitate airflow, yet prevent leakage of electrolyte due to electrolyte penetration of these large pores even under low air pressure. Further, leakage of electrolyte is very likely to occur through joints of the material and the sensor housing because most gas permeable materials are made of PTFE which does not bond well to plastic surfaces.
Another major problem with oxygen sensors is motion sensitivity. Electrolyte near the counter electrode has a higher than normal concentration of dissolved oxygen, since oxygen is generated at the counter electrode. When the sensor is moved or its orientation changed, the electrolyte moves inside the electrolyte reservoir. Contact of the oxygen-rich electrolyte with either the working or reference electrode causes a significant change in electrode potential, and hence, a possible drastic change in sensor output.
In order to overcome the motion sensitivity problem, U.S. Pat. No. 6,024,853 describes a sensor in which at least one protective electrode is used. The protective electrodes are disposed around the peripheral surface of the working electrode, and are held at approximately the same potential as that of the working electrode so that the majority of dissolved oxygen can be reduced before reaching the working electrode.
The sensor, is however, much more difficult to manufacture because precise positioning of the protective electrodes is critical to achieve good performance. Since the protective electrodes must be placed close to the working electrode, electrical short-circuiting may occur. Furthermore, the sensor requires more complicated circuitry to operate than a three electrode sensor, which adds additional cost to the associated gas detection instrument.
It is therefore an object of the invention to provide a compact, long-lived oxygen sensor that is free of electrolyte leakage.
It is another object of the invention to provide a leakage-free sensor that is as accurate as a conventional, galvanic-type sensor, and that can be operated constantly in a wide range of relative humidity.
It is a further object of the invention to provide a leakage-free sensor that is a potentiostatic type, three-electrode sensor so that it is interchangeable with three-electrode toxic gas sensors in suitably designed gas detection instruments.
It is a still further object of the invention to provide an oxygen sensor that has minimal motion sensitivity, with sensor output substantially independent of changes in sensor positioning and orientation, and stable upon sensor movement.
To achieve these and other objects, the invention is directed to an electrochemical sensor for measuring atmospheric oxygen comprising:
a housing having disposed therein a working electrode, a reference electrode and a counter electrode, and an electrolyte disposed in a reservoir therefore, the reservoir including an air space over the electrolyte, each of the working electrode, reference electrode and counter electrode comprising a catalytic material disposed on a surface of a porous support;
a vent hole passing through the housing;
the counter electrode being mounted within the housing to seal the vent hole with a porous support surface facing the vent hole and a surface having catalytic material disposed thereon facing away from the vent hole; and
a gas communication means connecting the counter electrode to the air space in electrolyte reservoir.
The invention is thus directed to a gas sensor in a case having, typically, a small hole at opposite ends thereof, these holes being sealed by gas permeable electrodes. Within the case is a partially filled electrolyte reservoir, having an air space therein, which also has holes at opposite ends thereof. An absorbent material passes from one end of the sensor to the other end, immobilizing the electrolyte outside of the reservoir but maintaining electrical contact with the electrodes, and a solid gas communications means contacts the counter electrode and extends into the air space in the reservoir. This arrangement permits equalization of pressure through the sensor.
Generally, the oxygen sensor of the invention is of the three electrode-type, with a working electrode, a reference electrode and a counter electrolyte, the reference electrode preferably being disposed between the working electrode and the counter electrode. All the electrodes are conventional gas diffusion type electrodes made by depositing at least one noble metal catalyst impregnated with a hydrophobic binder on a porous, hydrophobic support membrane, and is therefore gas permeable. A dilute acidic or basic aqueous solution is employed as electrolyte, which not only provides ionic conductivity between the electrodes, but also participates in electrode reactions.
The sensor of the invention includes a capillary gas access hole near the working electrode which admits gas to the sensor for detection, and which controls sensor output by limiting the flux of gas to the working electrode. Apart from this capillary hole, there is an additional gas vent opening in the sensor housing, which is made distant from the gas access hole to avoid affecting sensor operation. The counter electrode of the sensor is constructed and arranged to act as a sealing means for the gas vent opening so that electrolyte will not leak out and at the same time, oxygen generated at the electrode quickly diffuses through the gas vent opening to the outside atmosphere through the porous backing support of the counter electrode.
According to the invention, a gas communicating means is employed in conjunction with the counter electrode to balance air pressure within the cell. The gas communicating means is made of a porous, hydrophobic, chemically resistant solid material, and is disposed in such a way that it has not only significant presence in most of the empty spaces in the sensor but also has solid contact with the counter electrode.
The sensor has a large electrolyte reservoir partially charged with electrolyte. A substantial empty space is available for electrolyte expansion in the event of humidity uptake by the electrolyte under conditions of prolonged exposure to high relative humidity. A hydrophilic, chemically resistant absorbent material is packed between the electrodes to immobilize electrolyte therein. In most cases the electrolyte is able to move freely in empty spaces. According to the invention, an electrolyte separator is disposed between the reference electrode and the counter electrode, the separator having a large area so that it substantially separates the electrolyte in the working-reference electrode stack from the electrolyte in the remainder of the cell, while allowing contact of the two electrolytes through a small ionic pathway to maintain electric continuity. The electrolyte separator stops bulk flow of electrolyte in the electrolyte reservoir toward both working and reference electrodes, thus making the sensor output relatively independent of sensor movements, and changes in sensor positioning and orientation.